I want to modify the appearance of the first display of an acronym. I have tried:

\usepackage[toc,xindy]{glossaries}
\newcommand*{\glossfirstformat}[1]{\textit{#1}}
\defglsentryfmt{\glossfirstformat{\glsgenentryfmt}} %Does work
\defglsentryfmt[\acronymtype]{\glossfirstformat{\glsgenentryfmt}} %Does not have any effect

Addendum:

It seems like \acronymtype is only defined if the option acronym is used to the glossariespackage. But I want to have only one glossary including acronyms. I use the trick

What do you want to display when the acronym is used for the first time? It's not clear from your question. Also, is the change supposed for a single acronym or for all of them?
– karlkoellerMar 12 '15 at 10:28

@karlkoeller All of them, and the full name in italics.
– user877329Mar 12 '15 at 10:41

If you don't use the acronym package option, \acronymtype defaults to the main glossary so \defglsentryfmt[\acronymtype]{...} becomes equivalent to \defglsentryfmt{...}. Do you want something like this example?
– Nicola TalbotMar 14 '15 at 17:43